Process for treating biomass

ABSTRACT

This invention is directed to a process for treating biomass. The biomass is treated with a biomass swelling agent within the vessel to swell or rupture at least a portion of the biomass. A portion of the swelling agent is removed from a first end of the vessel following the treatment. Then steam is introduced into a second end of the vessel different from the first end to further remove swelling agent from the vessel in such a manner that the swelling agent exits the vessel at a relatively low water content.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under CooperativeAgreement No. DE-EE0005071 awarded by the United States Department ofEnergy. The Government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to a process for treating biomass using aswelling agent. In particular, this invention relates to a process fortreating biomass using a swelling agent and recovering a substantialportion of the swelling agent in relatively pure form.

BACKGROUND OF THE INVENTION

Biomass is generally composed of cellulose, hemicellulose, and lignin.The cellulose portion of biomass is particularly desirable in that thisportion can be converted into individual sugar components, which canthen be converted by microorganisms into various useful chemicalcompounds. As one example, the individual sugar components can beconverted by microorganisms into ethanol, which can be used as a fuel orfuel additive for combustion engines.

The cellulose and hemicelluloses portions of the biomass are tightlybound to the lignin. Unless the cellulose and hemicelluloses are atleast somewhat unbound from the lignin, conversion to individual sugarcomponents, and into various chemicals by microbial action, can behighly inefficient. Biomass treatment processes such as dry milling, wetmilling, steam explosion and ammonia fiber expansion (AFEX™—pending useand registration by MBI), however, have been used to unbind or breakapart the various portions of the biomass, and thereby increaseefficiency of conversion to individual sugar components, and ultimatelyto the various chemicals produced by microbial action.

U.S. Pat. No. 7,915,017 (Dale), for example, discloses a biomasstreatment process that renders the structural carbohydrates of thebiomass more accessible and/or digestible. The process uses ammonia toswell or rupture the biomass. The ammonia is recovered as ammonia vaporfrom an ammonia column, ultimately recycling the recovered ammonia inrelatively dry form.

U.S. Pat. No. 5,171,592 (Holtzapple) discloses an ammonia biomasstreatment process. The process includes recovery and recycle of theammonia using superheated ammonia vapor to strip residual ammonia fromthe treated biomass.

AFEX™ type processes have been found to be quite effective in treatingbiomass. However, recovery of the various swelling agents used in thoseprocesses have been less than desirable in that substantial steps havebeen involved to recover the swelling agents for reuse. In order toincrease efficiencies of the overall treatment process, it is thereforedesirable to increase the efficiency of recovery and reuse of theswelling agents.

SUMMARY OF THE INVENTION

This invention provides a process for treating biomass that is highlyeffective in treating the biomass. The invention includes the use of aswelling agent as a treatment agent. The overall treatment process ishighly efficient and is particularly efficient in the recovery and reuseof the swelling agent. The swelling agent can be recovered in arelatively pure form such that downstream processing and/or recovery ofthe swelling agent for recycle is minimal.

According to one aspect of this invention, there is provided a processfor treating biomass. The process comprises a step of treating thebiomass with a biomass swelling agent within a vessel to swell orrupture at least a portion of the biomass. At least a portion of theswelling agent is removed from a first end of the vessel, while leavingat least a majority of the treated biomass within the vessel. Thetreated biomass left within the vessel is then contacted with steam,which is introduced into a second end of the vessel different from thefirst end, to further remove swelling agent from the vessel.

In one embodiment, the contact of the steam and the treated biomass inthe vessel is carried out such that the vessel is at a bed angle of notgreater than 30 degrees from vertical.

Alternatively, the biomass is contained within the vessel at a bedporosity of at least 85 vol %.

Numerous swelling agents can be used according to this invention. Oneexample of an effective swelling agent is a nitrogen containingcomposition, e.g., ammonia.

The swelling agent is added to the vessel in a quantity that will swellor rupture the biomass. For example, the swelling agent can be added toprovide a weight ratio of swelling agent to biomass in a contacting zoneof the vessel of at least 0.1:1.

In another embodiment of the invention, the contact of the steam and thetreated biomass in the vessel is carried out based on a desired orpredetermined Archimedes number. For example, the contact of the steamand the treated biomass in the vessel can be carried out to provide anArchimedes number of at least 4.

The swelling agent is removed from the vessel at a low water content.For example, the swelling agent can be removed from the vessel in afirst step by a letting down pressure in the vessel and a second step ofcontacting the biomass with steam, with the recovered swelling agenthaving a low water content. The swelling agent can be removed andrecovered directly from the treatment vessel during the steam contactingstep. The directly recovered swelling agent can have a water content ofnot greater than 15 wt %, based on total content of swelling agentrecovered during the steam contacting step. The swelling agent can beremoved directly from the vessel as a vapor.

The contact zone of the vessel preferably has a length greater than itswidth. For example, the biomass can be contained within a contact zoneof the vessel such that the contact zone has a length to diameter ratioof greater than four.

Contacting or stripping steam can be supplied to the vessel over a widerange of temperatures and pressures. In other words, the steam suppliedto the vessel to remove residual swelling agent can be saturated orsuperheated steam.

The biomass is treated with the swelling agent in a manner that swellsor ruptures the biomass, leaving a biomass that can be more easilydigested by enzyme action. For example, the biomass can be treated withswelling agent to increase enzyme digestibility by at least 10% relativeto that initially supplied to the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of various preferred embodiments of this invention are shown inthe attached Figures, wherein:

FIG. 1 is a simple flow diagram of one embodiment of recovering swellingagent as a vapor, in which the swelling agent vapor is exemplified asammonia vapor;

FIG. 2 is a standard ammonia-water binary mixturedensity-temperature-composition diagram, which can be used to determineArchimedes number of the treatment vessel in accordance with oneembodiment of this invention; and

FIG. 3 is a plot of one example of the quality of recovery of swellingagent compared to the amount of swelling agent that can be recoveredaccording to this invention.

DETAILED DESCRIPTION OF THE INVENTION I. OVERALL PROCESS FOR TREATINGBIOMASS

This invention relates to a process for treating biomass, i.e.,cellulosic biomass, using a swelling agent. Treating the biomass withthe swelling agent increases the chemical and biological reactivity ofbiomass for subsequent processing. For example, contacting the biomasswith the swelling agent can cause the biomass to swell or rupture,increasing the chemical and biological reactivity of the biomass forsubsequent processing.

The invention represents a significant improvement over known treatmentprocesses by effectively recovering the swelling agent directly from thetreatment vessel and recycling the swelling agent for reuse. Theinvention is particularly effective in that the swelling agent can berecovered in substantially pure vapor form, i.e., little to no moistureor water content, when steam is used as a driving force to evacuate thetreatment vessel of residual swelling agent following treatment of thebiomass with the swelling agent to swell and/or rupture at least aportion of the biomass.

II. BIOMASS

Biomass refers to living and recently dead biological material that canbe used as fuel or for industrial production. Generally, biomass refersto plant matter grown for use as biofuel, but it also includes plant oranimal matter that can be used for production of fibers, chemicals orheat. Biomass may also include biodegradable wastes that can be burnedas fuel. It excludes organic material which has been transformed bygeological processes into substances such as coal or petroleum.

Particularly suitable biomass includes such plant matter containing arelatively high content of cellulose. Examples of such biomass or plantmatter include miscanthus, switchgrass, wheat straw, rice straw, oathulls, hemp, corn (e.g., stover or cob), poplar, willow, sugarcane andoil palm (palm oil). Even municipal wastes such as newspaper can all beused as suitable biomass material.

Other examples of biomass include stems, leaves, hulls, husks, wood,wood chips, wood pulp, and sawdust. Particular examples of paper wasteinclude discard photocopy paper, computer printer paper, notebook paper,notepad paper, typewriter paper, newspapers, magazines, cardboard, andpaper-based packaging materials.

In one embodiment, the biomass is predominantly one or more C₄ grasses.C₄ grasses are classified by their pathway of carbon dioxide metabolism,which involves intermediates with 4 carbon atoms. This is described inBiology of Plants, by Raven, Evert, and Curtis, Worth Publishing Co.,second edition, 1976, pages 116-117, incorporated herein by reference.Particularly preferred C₄ grasses are C₄ perennial grasses. Perennialgrasses do not require yearly planting and fertilization and aretherefore more suitable for fermentation and ethanol production thanannual grasses. Particularly preferred C₄ perennial grasses includeswitchgrass, miscanthus, cord grass, and rye grass. These grasses areparticularly fast growing. Cord grass is classified as a C₄ grass eventhough a portion of its growth cycle uses C₃ metabolism.

III. SWELLING AGENT

The swelling agent is a chemical composition effective in swellingand/or rupturing at least a portion of the biomass in the contact ortreatment zone of a vessel. The treated biomass is a highly desirablefeed for fermentation, as the treated biomass will have a significantamount of cellulose available for fermentation compared to the untreatedbiomass. Fermentation can be anaerobic (deficient in oxygen) as well asaerobic (oxygenated). Under aerobic conditions, microorganisms such asyeast cells can break down sugars to end products such as CO₂ and H₂O.Under anaerobic conditions, yeast cells utilize an alternative pathwayto produce CO₂ and ethanol. The fermentation reaction of the presentinvention is preferably anaerobic, i.e., partially or completelydeficient in oxygen. Fermentation can also be used to refer to the bulkgrowth of microorganisms on a growth medium where no distinction is madebetween aerobic and anaerobic metabolism.

As a part of the fermentation process, the treated biomass can becontacted with one or more cellulase enzymes in an aqueous mixture. Thecellulase can be provided as a purified enzyme or can be provided by acellulase-producing microorganism in the aqueous mixture. Cellulase caninclude any enzyme that effects the hydrolysis or otherwise solubilizescellulose (including insoluble cellulose and soluble products ofcellulose). Suitable sources of cellulase include such commercialcellulase products as Spezyme™ CP, Cytolase™ M104, and Multifect™ CL(Genencor, South San Francisco, Calif.).

Examples of swelling agents include but are not limited to: 1) watersoluble amines having the structure NRR¹R² where R, R¹ and R² are eitherthe same or different and are selected from the group consist of H orhydrocarbons containing 1-20 carbons (alternatively 1-10 carbons or 1-8carbons or 1-6 carbons), optionally substituted with oxygen, nitrogen,sulfur or phosphorous, or where two or more of the R groups are attachedto form a cyclic group (particular examples include ammonia, methylamine, dimethylamine, N-methyl, ethylamine, tripropylamine, andmorpholine); 2) water soluble ammonium ions having the structure+NRR¹R²R³ where R, R¹, R² and R³ are either the same or different andare selected from the group consisting of H or hydrocarbons containing1-20 carbons (alternatively 1-10 carbons or 1-8 carbons or 1-6 carbons),optionally substituted with oxygen, nitrogen, sulfur or phosphorous, orwhere two or more of the R groups are attached to form a cyclic group(particular examples include, ammonium hydroxide, ammonium chloride, andtrimethylammonium chloride); 3) hydroxides, carbonates, and bicarbonatesof lithium, sodium, potassium, magnesium, and calcium, such as sodiumhydroxide, magnesium carbonate, and calcium carbonate (lime); 4) watersoluble mono, or poly carboxylic acids containing 1-20 carbons(alternatively 1-10 carbons or 1-8 carbons or 1-6 carbons) such ascarbonic, acetic, trifloroacetic, succinic and citric; 5) inorganicacids such as sulfuric, sulfurous, nitric, nitrous, phosphoric, andhydrochloric, including agents that form inorganic acids when dissolvedin water such as sulfur dioxide, which forms sulfurous acid whendissolved in water.

In one example, the swelling agent is in vapor form at one atmospherepressure. In particular, the swelling agent is a vapor at one atmospherepressure, and at least some portion (e.g., at least 20 wt % or at least40 wt % or at least 60 wt %) of the swelling agent is a liquid withinthe vessel as the swelling or rupturing phase of the process is carriedout.

The amount of swelling agent for treating the biomass can depend on theparticular swelling agent used, with the particular biomass beingtreated having an effect in some cases as well. In general, the amountof swelling agent added to the treatment portion of the vessel is at alevel sufficient to swell and/or rupture a desired portion of thebiomass. For example, swelling agent is added to the vessel to provide aweight ratio of swelling agent to biomass in the treatment portion ofthe vessel of at least 0.1:1; alternatively from 0.1:1 to 10:1, or from0.1:1 to 5:1.

IV. VESSEL

The biomass can be treated in any vessel suitable for contacting thebiomass with vapor and liquid compositions in order to effectively swelland/or rupture the biomass. Preferably, the vessel is a tube or hollowcylinder. In a particular embodiment, the vessel is a tube or hollowcylinder having ports located near its axis at either end to permit flowof influent and effluent gas and/or liquid streams.

In one embodiment of the invention, the vessel has an internal axiallength greater than its internal diameter. For example, the vessel canhave an internal axial length at least four times greater than itsinternal diameter, alternatively at least eight times greater than itsinternal diameter, or at least ten times greater than its internaldiameter, or at least twelve times greater than its internal diameter.

The vessel can be arranged as part of a batch or continuous processsystem. There can be only one vessel, although more than one vessel canbe used. In embodiments that incorporate more than one vessel in whichbiomass material is contained, the vessels can be arranged in parallelor series.

V. PROCESS CONDITIONS

Biomass material can be retained as a static or moving bed within thevessel. In the form of a static bed, the biomass can be retained bymeans of a permeable support, such as support mesh or screens fitted atthe influent and effluent ends of the vessel. Mesh or screens withopenings smaller than the biomass particles, preferably at least fivetimes smaller, and most preferably at least ten times smaller, are used.At each end of the vessel, a plenum is preferably provided to allow foreven gas flow distribution throughout the bed of biomass particles.

The biomass material is preferably arranged in the vessel as a porouspacked bed. In this arrangement, the biomass is in particulate form, inwhich each particle is fixed in position relative to one another.

The biomass is also arranged in the vessel such that it is effectivelytreated with the swelling agent to cause the biomass to swell and/orrupture. The biomass is also arranged in the vessel such that theswelling agent can be removed with steam in such an manner that theswelling agent can be recovered directly from the vessel with little tono water content.

A variety of parameters can be used to ensure appropriate removal of alow moisture content swelling agent from the vessel following treatmentof the biomass in the vessel. An example of one such parameter isArchimedes number. The Archimedes number (N_(Ar)) quantifies the ratioof free to forced convection in a fluid system. N_(Ar) is a usefulparameter for controlling steam flow rate at levels that permit recoveryof the swelling agent as substantially pure vapor, i.e., little to nomoisture content. The value of N_(Ar), calculated from measurablequantities as described below, indicates the relative strengths of freeand forced convection within the mass transfer zone of the bed ofbiomass particles, which determines the fraction of residual swellingagent that may be recovered as substantially pure vapor.

N_(Ar) can be calculated from Reynolds number (N_(Re)) and Grashofnumber (N_(Gr)) as:

N _(Ar) =N _(Gr) /N _(Re) ².

For a cylindrical porous packed bed with circular cross section, theGrashof number (N_(Gr)) can be calculated from the bed diameter D_(Bed)and bed angle θ_(Bed), the steam and vapor temperatures T_(steam) andT_(vap), densities ρ_(steam) and ρ_(vap), and viscosities μ_(steam) andμ_(vap), and the gravitational constant g=9.81 m/s² as:

N _(Gr)=[2g D _(Bed) ³(ρ_(steam)+ρ_(vap))²(T _(steam) +T_(vap))cos(θ_(Bed))]/[(T _(steam) +T _(vap)) (μ_(steam)+μ_(vap))²].

For axial steam flow through a cylindrical porous packed bed withcircular cross section, the Reynolds number (N_(Re)) can be calculatedfrom the bed diameter D_(Bed), the steam mass flow rate m_(steam), andsteam dynamic viscosity μ_(steam) as:

N _(Re)=(4 m_(steam))/(πμ_(steam)D_(Bed)).

Bed porosity (ε_(Bed), volume %) may be calculated using the bed densityρ_(Bed) and the biomass true density ρ_(True), asε_(Bed)=100(1−ρ_(Bed)/ρ_(True)). In one embodiment, biomass particlesare included in the treatment vessel at a bed porosity of at least 85vol %, alternatively at least 88 vol %.

Biomass bed density (ρ_(Bed), kg/m³)may be calculated asρ_(Bed)=m_(Bed)/V_(Bed), where V_(Bed) is the bed volume (m³) calculatedfrom the bed dimensions using a suitable geometric formula.

Biomass bed dry mass (m_(Bed), kg) can be calculated from m_(total) andx_(moist) as m_(Bed)=m_(total)(1−x_(moist)/100).

Steam density (ρ_(steam), kg/m³) and viscosity (μ_(steam), kg/m s) maybe interpolated from standard literature steam table values using themeasured steam temperature T_(steam).

Vapor composition, density (ρ_(vap), kg/m³), and dynamic viscosity(μ_(vap), kg/m s) may be calculated from the vapor temperature T_(vap)by interpolation using FIG. 2, or by comparison to literature data forproperties of saturated ammonia-water vapor mixtures at the appropriatepressure Tillner-Roth, R., and D. G. Friend, “A Helmholtz free energyformulation of the thermodynamic properties of the mixture{water+ammonia}”, J. Phys. Chem. Ref. Data, Vol. 27, no.1, 1998, p63-83, the contents of which are incorporated herein by reference.

“Steam mass rate (m_(steam), kg/s)” refers to the mass of steam that isinjected or input to the reaction or contact vessel. The steam mass ratemay be measured using an appropriate flow meter, such as a vortex meter.Alternatively, the steam mass rate may be determined by measuring thecomposite moisture content of a biomass bed before and after steamingfor a measured interval of time, during which negligible steam flow fromthe exit end of the bed is observed.

“Steam and vapor temperatures (T_(steam) and T_(vap), K)” refer to steamtemperature at the entrance of the reaction or contact vessel andtemperature of swelling agent exiting from the reaction or contactvessel, with the swelling agent exiting from the vessel substantially inthe vapor phase. The steam and vapor temperatures may be measured usingtemperature sensors positioned in the inlet steam and exit streams nomore than one bed diameter from the inlet and exit ends of the bed,respectively.

The steam that is used as the driving force to evacuate the treatmentvessel of residual swelling agent can be saturated or superheated. Thesteam pressure will depend upon the pressure rating of the treatmentvessel. As an example, steam pressures of from 5 psig to 500 psig can beused. Alternatively steam pressures from 5 psig to 400 psig or from 5psig to 300 psig can be applied.

“Biomass true (skeletal) density (ρ_(True), kg/m³)” refers to the truedensity of biomass in the reaction portion of a vessel, which may bemeasured by volumetric pycnometry analysis. True density values weremeasured using an automatic gas pycnometer instrument (QuantachromePentapyc 5200e) following the standard methods.

“Bed angle (θ_(Bed))” refers to the angle between the direction of axialflow through the biomass bed and the local gravity vector. θ_(Bed) maybe conveniently measured using a carpenter's plumb line or spirit level.In one embodiment, the bed angle is not greater than 30 degrees fromvertical, alternatively not greater than 25 degrees from vertical, ornot greater than 20 degrees from vertical.

“Biomass moisture content (x_(moist), mass %)” refers to the watercontent of the biomass in the vessel, and may be measuredthermo-gravimetrically as loss on drying at 105° C., as described inEhrman, T., “Standard method for determination of total solids inbiomass”, NREL Laboratory Analytical Procedure LAP-001, NationalRenewable Energy Laboratory, Golden, Colo., November 1994, the contentsof which are incorporated herein by reference.

“Biomass total bed mass (m_(total), kg)” refers to total biomass in thereaction portion of a vessel (i.e., zone in which biomass is treated),and may be measured by gravimetrically measuring the difference betweenthe full and tare masses of a container to be assembled as the bed inthe vessel. Equivalently, m_(total) may be measured as the differencebetween the full and tare masses of a bin from which biomass istransferred with minimal mass loss into the reaction portion of thevessel.

In order to remove and recover swelling agent from the treated biomassat the desired moisture content, the Archimedes number should be highenough so that the recovered swelling agent can be recycled for reuse,with little to no processing, since the recovered swelling agent is in ahighly pure form. For example, Archimedes number can be at least 4,alternatively at least 4.5, or at least 5.

The swelling agent can be recovered from the reactor or treatment vesselwith very little water. For example, the swelling agent can be recovereddirectly from the treatment vessel following biomass treatment and steamcontacting or stripping at a water content of not greater than 10 wt %,alternatively not greater than 5 wt % water, alternatively not greaterthan 3 wt % water, or not greater than 1 wt % water, based on totalweight of swelling agent recovered directly from the treatment vesselfollowing biomass treatment and steam contacting or stripping. Theswelling agent can be also be recovered directly from the treatmentvessel, and during the steam contacting or stripping step, at a watercontent of not greater than 15 wt %, alternatively not greater than 10wt % water, or not greater than 5 wt % water, based on total weight ofswelling agent recovered directly from the treatment vessel during steamcontacting or stripping. The swelling agent can be recovered directlyfrom the vessel as a vapor.

The temperature and pressure of the vessel during treatment of thebiomass with the swelling agent are sufficiently high to enhanceswelling and/or rupturing of the biomass. When a swelling agent is usedthat is a vapor at standard conditions, i.e., one atmosphere and 25° C.,it is preferred to maintain the temperature and pressure of the vesselsuch that at least a portion of the swelling agent is in liquid phase asthe swelling or rupturing phase of the process is carried out.

In one embodiment of the invention, the process is carried out such thatthe vessel containing the biomass is within a temperature range of from25° C. to 200° C. Preferably, the process is carried out such that thetemperature of the vessel containing the biomass is within a range offrom 30° C. to 180° C., more preferably from 50° C. to 150° C.

In another embodiment of the invention, the process is carried out suchthat the pressure of the vessel during treatment of the biomass with theswelling agent is within a range of from 20 psia (137.9 kPaa) to 1000psia (6895 kPaa). Preferably, the treatment portion of the process iscarried out such that the pressure of the vessel containing the biomassbeing treated is within a range of from 40 psia (275.8 kPaa) to 800 psia(5516 kPaa), more preferably from 60 psia (413.7 kPaa) to 500 psia (3447kPaa).

The biomass is contacted or treated with the swelling agent for a timethat is sufficient to swell and/or rupture at least a portion of thebiomass. For example, the biomass can be contacted or treated withswelling agent to swell and/or rupture at least 25 wt % of the biomassmaterial or at least 50 wt % of the biomass material. The biomass can becontacted with swelling agent for at least one minute, alternatively forat least two minutes, or at least five minutes.

VI. REMOVING SWELLING AGENT

Following treatment of the biomass with the swelling agent, at least aportion of the swelling agent is removed from a first end of the vessel,while leaving at least a majority of the treated biomass within thevessel. The treated biomass left within the vessel is then contactedwith steam that is introduced into a second end of the vessel differentfrom the first end to remove at least a portion of residual swellingagent, i.e., swelling agent that has not been absorbed into the biomass,remaining in the vessel.

The swelling agent can be removed from the vessel following treatment byopening the first end of the vessel in a manner that reduces pressurewithin the vessel. For example, a valve can be placed in a line from thefirst end of the vessel such that the valve can be opened as desired toremove swelling agent from the treated biomass. As a particular example,the pressure of the vessel can be reduced to less than 50% of thepressure of the vessel during treatment with the swelling agent.Alternatively, the pressure of the vessel can be reduced to less than20% or less than 10% of the pressure of the vessel during treatment withthe swelling agent, in order to remove swelling agent from the treatedbiomass in the vessel.

Residual swelling agent can remain in the vessel following the openingof the first end of the vessel to remove swelling agent from the vessel.This residual swelling agent can then be removed by injecting steam intothe second end of the vessel. This injection is carried out under theconditions as described above in order to recover a substantial portionof the swelling agent in relatively pure or dry form.

As the steam is injected into the second end of the vessel, the firstend of the vessel can be monitored to determine the quality of thestream exiting the second end. For example, the stream exiting thevessel can be monitored to determine the content of the swelling agentand moisture level of the stream exiting the vessel as the steam isapplied at the second end of the vessel. As long as the stream exitingthe vessel is at an acceptably low moisture content, the stream can berecovered for re-use as the recovered stream will be highly pureswelling agent.

VII. FURTHER PROCESSING OF BIOMASS

In certain embodiments of the invention, the treated biomass materialcan be further processed or otherwise chemically converted. For example,the biomass material can be removed following steam application toremove residual swelling agent from the vessel, and then be used toproduce chemical derivates such as to make ethanol or other compositionsuseful as transportation fuel or fuel components. In such an example,the biomass material can be fermented to produce the desired fuelcomponents.

In a preferred embodiment of the invention, the swelling agent cansufficient treat the biomass material to increase enzyme digestibilityof the biomass. Enzyme digestibility refers to the ability of thebiomass material to be converted into its constituent sugar componentsby hydrolytic enzymes. According to this invention, enzyme digestibilityis measured using a standard laboratory protocol. See, e,g., “Enzymaticsaccharification of lignocellulosic biomass,” National Renewable EnergyLaboratory Technical Report TP-510-42629, March 2008.

For the purposes of this invention, enzyme digestibility is the rate atwhich a biomass material may be digested to one or more of glucose andxylose using cellulase or xylanase enzymes. The quantitative effect ofcontact or treatment of the biomass material with the swelling agentwith regard to the enzyme digestibility of the biomass material can bemeasured by adding 0.15 grams dry mass of biomass sample that has beencontacted or treated with the swelling agent to a glass scintillationvial, and the same dry mass of biomass that has not been contacted ortreated to a separate vial. Preferably, duplicate vials of both biomasssamples are prepared.

To each vial is then added 5.0 milliliters of 0.1 molar pH 4.8 sodiumcitrate buffer, and a quantity of cellulase enzyme solution equal to 1.5Filter Paper Units (FPU) of activity. An equal quantity of xylase enzymecan also be added to each vial.

The total volume in each vial is then diluted to 10.0 milliliters byadding distilled water. The vials are then sealed and incubated at 50±1°C., with sufficient agitation to keep the solids suspended, for 48hours. Conditions during the incubation period, including temperatureand pH, can be adjusted as necessary to suit the particular enzymes usedin digestion.

After incubation, a liquid aliquot is drawn from each vial and filteredthrough a 0.45 micron filter, and the filtrate is then analyzed todetermine the concentration of glucose, xylose, or both using HPLC orany other suitable quantitative technique.

Increased enzyme digestibility of biomass can then be calculated asXcell=100*[(CGlu)abs/(CGlu)−1], where Xcell is the percent increase incellulose digestibility, (CGlu)abs the concentration of glucose in thefiltrate from the biomass that has been contacted or treated with theswelling agent, and (CGlu) is the concentration of glucose in thefiltrate from the biomass that has not been contacted or treated withthe swelling agent.

A similar calculation can be performed for increased enzymedigestibility based on the concentrations of xylose in the filtratealiquots. In such a case, Xcell=100*[(CXyl)abs/(CXyl)−1], where Xcell isthe percent increase in xylan digestibility, (CXyl)abs is theconcentration of xylose in the filtrate from the biomass that has beencontacted or treated with the swelling agent, and (CXyl) is theconcentration of xylose in the filtrate from the biomass that has notbeen contacted or treated with the swelling agent. Preferably, contactor treatment of the biomass material with the swelling agent increasesenzyme digestibility by at least 5%, more preferably by at least 50%,and most preferably by at least 100%. This increase can be measured byone or more of an increase in cellulose and xylose digestibility asdefined by Xcell.

In one embodiment of the invention, the biomass is treated with theswelling agent to increase enzyme digestibility by at least 10% relativeto that initially supplied to the vessel. Alternatively, the biomass istreated with the swelling agent to increase enzyme digestibility by atleast 20%, more preferably at least 30%, and most preferably at least40%, relative to that initially supplied to the vessel.

VIII. EXAMPLES

FIG. 1 shows a flow diagram of recovery of swelling agent vapor,exemplified as ammonia vapor, in which ammonia vapor is expelled fromthe bottom of a vertical porous packed bed of biomass at atmosphericpressure, as steam is introduced to the top of the bed. At any pointduring the recovery process, the bed can be divided into three zones. Atthe top of the bed is a stripped zone, where swelling agent, i.e., NH₃,concentration is negligible and the bed temperature is about equal tothat of the incoming steam. At the bottom of the bed is an NH₃ saturatedzone into which steam has not yet penetrated, where NH₃ concentration isrising and bed temperature is falling due to absorption of NH₃-richvapor flowing down from the sections above. Exiting the bottom of theNH₃ saturated zone is vapor whose composition is in equilibrium with theabsorbed liquid in the saturated zone.

Between the stripped zone and the NH₃ saturated zone is a mass transferzone, where steam is condensing, liberating heat that vaporizes ammoniafrom the absorbed liquid. Along the bottom edge of the mass transferzone, the absorbed liquid is NH₃-rich, so that the vapor arising fromthe biomass is significantly colder and denser than the steam enteringthe top of the mass transfer zone. This density difference, between thehot steam and the colder vapor arising from the NH₃-rich biomass, maycause significant buoyant effects during the steam stripping process.

FIG. 2 shows the composition and density of vapor arising from biomassover the range of temperatures encountered during NH₃ recovery (datafrom Tillner-Roth, R., and D. G. Friend, “A Helmholtz free energyformulation of the thermodynamic properties of the mixture{water+ammonia}”, J. Phys. Chem. Ref. Data, Vol. 27, no.1, 1998, p63-83). For reference, FIG. 2 also shows the density of saturated steamat atmospheric pressure. At the bottom of the mass transfer zone, theNH₃-rich vapor temperature can be less than 10° C., and the densitydifference Δρ between the incoming steam and the NH₃-rich vapor issignificant, while at the top of the mass transfer zone the temperatureapproaches that of the steam, and Δρ approaches zero.

Steam is transported into the mass transfer zone by forced convection.Forced convective mixing of incoming steam with the NH₃-rich vaporarising from the biomass will increase the length of the mass transferzone. The longer the mass transfer zone is relative to the length of thebed, the less will be the percentage of residual ammonia that can berecovered from the bed as substantially pure vapor. Free convection,driven by the density difference Δρ between the steam and the NH₃-richvapor, will oppose forced convective mixing by segregating the steamfrom the NH₃-rich vapor, preventing lengthening of the mass transferzone.

Examples 1-8 General Experimental

A series of experiments was carried out in which biomass was milled,screened and packed into six identical cylindrical containers, each 9.7centimeters in diameter and 15 centimeters long. The containers wereconstructed with end plates made of perforated sheet with 41% open areato allow axial flow of vapor in and out of each container, and thebiomass was compressed into the containers. The six containers wereassembled into a vertical stainless steel tubular bed vessel 10.2centimeters outer diameter and 125 centimeters long, with the end ofeach container in contact with the adjacent containers, forming asegmented porous bed 9.7 centimeters in diameter and 90 centimeterslong.

The bed was pre-steamed by introducing steam at a mass flow rate of 1gram per second into the top of the tube and allowing steam anddisplaced air to flow vertically downward out the bottom of the bed tovent for approximately ten minutes. The bed was then ammoniated byblocking flow from the bottom of the bed while introducing compressedanhydrous ammonia vapor to the top of the bed. Maximum pressure duringammoniation reached 200 psig. Ammonia was added until a ratio of 1:1ammonia:dry biomass was achieved.

Following treatment of the biomass with the ammonia, the pressure wasreleased from the bed by allowing vapor to flow out the bottom of thebed until the bed pressure was 1 atmosphere. The residual ammonia wasthen recovered from the bed by introducing steam at a mass flow rate of1 gram per second into the top of the bed while allowing vapor to flowat atmospheric pressure from the bottom of the bed.

The temperature of the vapor expelled from the bottom of the bed wasmonitored using a temperature sensor positioned less than 10 centimetersfrom the bottom of the bed; the composition of the vapor was determinedfrom the temperature by interpolation using the NH₃—H₂O vaporcomposition curve in FIG. 2. The vapor was trapped in discrete citricacid fractions of known mass and concentration; the pH change was usedto calculate the mass of ammonia trapped in each fraction.

The following Table provides the various biomass characteristics andoperating conditions for each Example. Examples 1 and 2 are provided ascomparative examples. Examples 3-8 represent one or more embodiments ofthe overall invention. As an example of quality of recovery compared toamount of swelling agent that can be recovered according to thisinvention, a plot of the composition of the vapor expelled during steamstripping and recovery of the residual ammonia from the bed for Example3 is shown in FIG. 3.

TABLE Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Biomass type CornCorn Corn Corn Corn Wheat Wheat Wheat stover stover stover stover stoverstraw straw straw Mill, particle size Knife, Knife, Knife, Hammer,Hammer, Knife, Knife, Knife, ½″ ½″ ½″ 1″ ½″ 4.7 mm 8 mm 30 mm Bed angle(θ_(Bed), deg.) 90 45 0 0 0 0 0 0 Bed porosity (ε_(Bed), vol %) 87 87 8790 85 87 90 92 Archimedes No. (N_(Ar)) 0.0 3.8 5.8 5.8 5.8 6.1 6.1 6.1Residual NH₃ recovery (mass %) 81 89 97 97 89 97 97 97 NH₃ recoveredduring steam stripping 28 34 64 91 43 69 85 89 containing >90% ammonia(mass %)

As seen from the Examples, the experiments that were carried out withinthe desired parameters (e.g., bed angle, bed porosity, Archimedes no.,etc.) resulted in recovery of the ammonia at very high purity. The samewould be expected for a variety of swelling agents, particularlyswelling agents that are in the vapor phase at standard conditions,i.e., one atmosphere and 25° C. One benefit of recovering swelling agentat relatively high purity is that the swelling agent can be recycledwith little to no downstream processing.

The principles and modes of operation of this invention have beendescribed above with reference to various exemplary and preferredembodiments. As understood by those of skill in the art, the overallinvention, as defined by the claims, encompasses other preferredembodiments not specifically enumerated herein.

1. A process for treating biomass, comprising: treating the biomass witha biomass swelling agent within a vessel to swell or rupture at least aportion of the biomass; removing at least a portion of the swellingagent from a first end of the vessel, while leaving at least a majorityof the treated biomass within the vessel; and contacting the treatedbiomass left within the vessel with steam introduced into a second endof the vessel different from the first end to further remove swellingagent from the vessel, wherein during the contact of the steam and thetreated biomass in the vessel, the vessel is at a bed angle of notgreater than 30 degrees from vertical.
 2. The process of claim 1,wherein the biomass is contained within the vessel at a bed porosity ofat least 85 vol %
 3. The process of claim 1, wherein the swelling agentis ammonia.
 4. The process of claim 1, wherein swelling agent is addedto the vessel to provide a weight ratio of swelling agent to biomass ina contacting zone of the vessel of at least 0.1:1.
 5. The process ofclaim 1, wherein the contact of the steam and the treated biomass in thevessel is carried out to provide an Archimedes number of at least
 4. 6.The process of claim 1, wherein the swelling agent that is removed fromthe vessel by contact with the steam is recovered directly from thetreatment vessel during the steam contacting step, and the directlyrecovered swelling agent has a water content of not greater than 15 wt%, based on total content of swelling agent recovered during the steamcontacting step.
 7. The process of claim 6, wherein the swelling agentthat is removed from the vessel is removed as a vapor.
 8. The process ofclaim 1, wherein the biomass is contained within a contact zone of thevessel, with the contact zone having a length to diameter ratio ofgreater than four.
 9. The process of claim 1, wherein the steam issupplied to the vessel as saturated or superheated steam.
 10. Theprocess of claim 1, wherein the biomass is treated with the swellingagent to increase enzyme digestibility by at least 10% relative to thatinitially supplied to the vessel.
 11. A process for treating biomass,comprising: treating the biomass with a biomass swelling agent within avessel to swell or rupture at least a portion of the biomass; removingat least a portion of the swelling agent from a first end of the vessel,while leaving at least a majority of the treated biomass within thevessel; and contacting the treated biomass left within the vessel withsteam introduced into a second end of the vessel different from thefirst end to further remove swelling agent from the vessel, wherein thecontact of the steam and the treated biomass in the vessel is carriedout to provide an Archimedes number of at least
 4. 12. The process ofclaim 11, wherein the biomass is contained within the vessel at a bedporosity of at least 85 vol %
 13. The process of claim 11, wherein theswelling agent is ammonia.
 14. The process of claim 11, wherein swellingagent is added to the vessel to provide a weight ratio of swelling agentto biomass in a contacting zone of the vessel of at least 0.1:1.
 15. Theprocess of claim 11, wherein during the contact of the steam and thetreated biomass in the vessel, the vessel is at a bed angle of notgreater than 30 degrees from vertical.
 16. The process of claim 11,wherein the swelling agent that is removed from the vessel by contactwith the steam is recovered directly from the treatment vessel duringthe steam contacting step, and the directly recovered swelling agent hasa water content of not greater than 15 wt %, based on total content ofswelling agent recovered during the steam contacting step.
 17. Theprocess of claim 16, wherein the swelling agent that is removed from thevessel is removed as a vapor.
 18. The process of claim 11, wherein thebiomass is contained within a contact zone of the vessel, with thecontact zone having a length to diameter ratio of greater than four. 19.The process of claim 11, wherein the steam is supplied to the vessel assaturated or superheated steam.
 20. The process of claim 11, wherein thebiomass is treated with the swelling agent to increase enzymedigestibility by at least 10% relative to that initially supplied to thevessel.